1. Field of the Invention
The present invention relates generally to a disk drive of perpendicular magnetic recording type, and more particularly to a magnetic disk drive with a structure for avoiding magnetic disturbance due to a DC magnetic field.
2. Description of the Related Art
In recent years, in the field of magnetic disk drives such as hard disk drives, a perpendicular magnetic recording method has been attracted public attention as technology of exceeding the limits of recording densities in a longitudinal magnetic recording method. As a disk drive that employs the perpendicular magnetic recording method, a disk drive using a double-layered disk recording medium (hereinafter referred to as the disk) has been commercialized.
The disk includes a substrate, a recording magnetic layer exhibiting magnetic anisotropy in the vertical direction and a soft magnetic layer interposed between the substrate and the recording magnetic layer. When a magnetic flux is generated from one of the magnetic poles of the magnetic head during a data recording operation, the soft magnetic layer allows passage of a part of the magnetic flux to the other magnetic pole. Thus, the soft magnetic layer has such a function as to support the recording operation of the magnetic disk.
A disk drive of the perpendicular magnetic recording type basically utilizes the mechanism used in the longitudinal magnetic recording type, except for the aforementioned double-layered disk recording medium. More specifically, the magnetic heads used in both types are substantially the same. The magnetic head of the disk drive includes a write head of an inductive type and a read head having a GMR (giant magnetoresistive) element. The read and write heads are generally separated from each other and mounted on one slider.
Recently, development of a disk drive of the perpendicular magnetic recording type revealed that strong magnetic disturbance occurs on a disk surface due to a DC (direct current) magnetic field generated from a DC magnetized region of the disk. It is presumed that a leakage magnetic field from the DC magnetized region influences the disk as DC magnetic disturbance.
In a process of manufacturing a disk, an initial magnetizing process of magnetizing the overall disk surface by a DC magnetic field (or a DC erase process) is executed prior to track formatting. Most of the DC magnetized region formed by the initial magnetizing process on the disk is demagnetized by the formatting for forming a number of tracks. However, the DC magnetized region in the initial state remains unchanged as a non-record region (a guard band where no data signal is recorded) provided between tracks.
It is presumed that the leakage magnetic field due to the DC magnetized region causes the following problems. The problems will be described in detail with reference to FIGS. 6 and 7.
FIG. 6 shows a state where a magnetic head 3 including a read head (a GMR element) 30 is located above a target track (a track to be accessed) 100A of a disk 1 in a disk drive of the perpendicular magnetic recording type. As described above, a DC magnetized region exists between the track 100A and adjacent tracks 100B. Therefore, a leakage magnetic field 50 from the DC magnetized region influences the magnetic head 3 near the track 100A as magnetic disturbance.
When the read head 30 included in the magnetic head 3 performs a reading operation to read data from the track 100A, the GMR element is influenced by the magnetic field due to the magnetic disturbance and the operating point thereof shifts. As a result, it is highly possible that phenomenon, such as asymmetry or saturation, occur in a read signal waveform output from the read head 30. In other words, the read head 30 may output a read signal whose quality is deteriorated, in which case a read error will easily occur.
In a disk drive of the longitudinal magnetic recording type, since the disk 1 is DC magnetized in the longitudinal direction of the disk 1, substantially no leakage magnetic field from the DC magnetized region is present on the surface of the disk 1 (see FIG. 7). Therefore, the read head 30 is not influenced by magnetic disturbance due to a leakage magnetic field from a DC magnetized region.
Magnetic disturbance from the DC magnetic region is a technical problem to be solved in order to commercialize a disk drive of the perpendicular magnetic recording system using a double-layered disk recording medium.
A method of forming a magnetized region with the polarity opposite to that of the recording magnetization between tracks is proposed as prior art relating to the perpendicular magnetic recording (for example, Jpn. Pat. Appln. KOKAI Publication No. 10-320705). However, this method is difficult to commercialize, since a disk must be subjected to a complicated process. In addition, it is not confirmed that the influence of magnetic disturbance on the read head is effectively suppressed.